The present invention relates to equipment and to a process for gas-phase polymerization of olefin(s) in a fluidized and optionally mechanically stirred bed, particularly with a device improving the withdrawal and the emptying of the polymer manufactured.
It is known to polymerize one or more olefins in the gaseous phase at a pressure which is higher than atmospheric pressure in a reactor with a fluidized bed and a vertical side wall, where polymer particles being formed are kept in the fluidized state above a fluidization grid by virtue of a reaction gas mixture containing the olefin(s) to be polymerized and traveling according to an upward stream. The polymer thus manufactured in powder form is generally withdrawn from the reactor by at least one side discharge conduit situated along the vertical wall of the reactor above the fluidization grid and is then subjected to a decompression and degassing stage. The reaction gas mixture leaving via the top of the fluidized-bed reactor is returned to the base of the latter under the fluidization grid through the intermediary of an external circulation conduit provided with a compressor. While being returned, the reaction gas mixture is generally cooled with the aid of at least one heat exchanger provided in the external circulation conduit so as to remove the heat produced by the polymerization reaction. The polymerization is carried out in the presence of a catalyst or of a catalyst system introduced into the fluidized bed. High-activity catalysts and catalyst systems, which have been known already for a number of years, are capable of producing large quantities of polymer in a relatively short time, thus avoiding a stage of removal of the catalyst residues in the polymer.
When the polymer is withdrawn from the reactor, it is found to be accompanied by the reaction gas mixture present under pressure in the reactor. It has been observed that the proportion of the reaction gas mixture accompanying the polymer which is withdrawn is generally high. This makes it necessary to provide large-sized devices for decompressing and degassing the polymer which is withdrawn, and an appropriate and costly device for recovering and recompressing the reaction gas mixture withdrawn with the polymer and for recycling most of this gas mixture into the polymerization reactor. Such devices generally include a gas lock for polymer recovery, connected to the side wall of the fluidized-bed reactor by a withdrawal conduit fitted with an isolation valve. The recovery gas lock may also be connected to a decompression and degassing chamber by a discharge conduit, itself fitted with an isolation valve. In most cases the withdrawal conduit leaves the side wall of the reactor in a direction that is perpendicular to the said wall, that is to say in a horizontal plane. It then reaches a recovery gas lock either while still remaining in the same horizontal plane, as described in U.S. Pat. No. 4,003,712, French Patent No. 2 642 429 or European Patent No. 188 125, or while descending vertically after having formed an elbow, as described in European Patent No. 71 430. The withdrawal conduit may also leave the side wall of the reactor while being directed downwards and while forming a relatively small angle with a horizontal plane, for example an angle of 18.degree. as shown in FIG. 1 of French Patent No. 2 599 991, and next reach a recovery gas lock while descending vertically after having formed an elbow. It has been observed that, in all cases, the proportion of the reaction gas mixture accompanying the polymer which is withdrawn is relatively high and results in the abovementioned disadvantages. To solve this problem, European Patent No. 71 430 proposes to add to the recovery gas lock both a vent conduit connecting the gas lock to the fluidized-bed reactor at a point situated above the withdrawal conduit and a second recovery gas lock connected to the first gas lock by a connecting conduit.
In their French Patent No. 2730999, filed on Feb. 24, 1995, the Applicants have since then proposed equipment and a process for gas phase polymerization of olefin(s) making it possible to avoid the abovementioned disadvantages. Described therein is equipment for the gas phase polymerization of olefin(s) including a reactor with a fluidized and optionally mechanically stirred bed and with a vertical side wall, provided at its base with a fluidization grid, a conduit for external circulation of a reaction gas mixture, connecting the top of the reactor to the base of the latter under the fluidization grid and including a compressor and at least one heat exchanger, and at least one conduit for withdrawing polymer, provided with an isolation valve and connecting the side wall of the reactor to a recovery gas lock equipped with a discharge valve, which equipment is characterized in that the withdrawal conduit leaves the side wall of the reactor while being directed downwards, with the result that any portion of the said conduit forms with a horizontal plane an angle A ranging from 35 to 90.degree..
While the equipment and the method indicated above have completely solved the problems which have been raised, the Applicants have found that the installation of the device described in their French Patent No. 2730999 could present some disadvantages. In fact, as shown in FIG. 1 of French Patent No. 2730999, the installation of this withdrawal device involves a positioning of the branch connection for the withdrawal conduit at a relatively high level of the reactor. This means that the polyethylene which is below the branch connection for the withdrawal conduit cannot be drawn off from the reactor. This polyethylene which cannot be withdrawn may represent considerable proportions of the total quantity of polyethylene present in the reactor. One of the solutions for removing this polyethylene could consist in increasing the relative height of the reactor in relation to the ground and, as a result, lowering the relative height of the branch connection for the withdrawal conduit in relation to the bottom of the reactor. Although technically viable, this solution would, however, be found to be extremely costly.
Equipment and a process for gas-phase polymerization of olefin(s) have now been found which make it possible to avoid the abovementioned disadvantages. In particular, they make it possible to withdraw and empty in an extremely simplified and efficient manner a polymer manufactured in a fluidized-bed reactor, considerably reducing the proportion of the reaction gas mixture withdrawn with the polymer, while avoiding blocking the withdrawal and emptying system, and while allowing easy installation in industrial plants.